AC-driven electroluminescence materials (devices) have been developed as flat-panel-type light-emitting light sources. As various electronic devices have been diversifying in recent years, electroluminescence devices have been used as display devices thereof. Further, they have also been used as decorative display materials. In addition to the above, large-sized flat panel displays have also been developed in-recent layers.
Electroluminescence materials (devices) are roughly classified into two types: particle-dispersion-type electroluminescence materials, wherein phosphor particles (electroluminescence particles) are dispersed in a highly dielectric substance; and thin-film-type devices, wherein a phosphor thin film is formed, mainly by vapor deposition, to be sandwiched between dielectric layers. The former particle-dispersion-type electroluminescence devices have characteristics that, for example, they can easily be made into a large area as a uniform area light source. However, they have problems that high voltage of 100 V or more is occasionally required for luminescence, and further, the luminance and efficiency are low (see, for example, “ELECTROLUMINESCENCE DISPLAY” (written by Toshio Inoguchi, published by Sangyo Tosho; July 1991), and “DISPLAY” (edited by Syunsuke Kobayashi and Kaichi Toyama, published by Maruzen Co., Ltd.; March 1993)).
In the particle-dispersion-type device, a luminescence layer, comprising phosphor powder contained in a high-dielectricity polymer, such as a fluorine-series rubber or a polymer having a cyano group, is arranged between a pair of electrically conductive electrode sheets, at least one of which is light-transmissible. In any ordinary embodiment of the particle-dispersion type, a dielectric layer is arranged, to prevent dielectric breakdown. The dielectric layer comprises powder of a ferroelectric substance, such as barium titanate, contained in a highly dielectric polymer. The phosphor powder used in this type generally comprises ZnS, as a host material thereof, which is doped with an appropriate amount of ions of Cu, Mn, Ce, Cl, Al, or the like.
Examples of an electron-trapping center, which is the so-called donor, include Cl and Al. Examples of a positive hole-trapping center, which is the so-called acceptor, include Cu and Zn holes. In the field of the present art, for example, luminescence between Cl or Al, which trapped electrons, and Cu, which trapped positive holes, is called DA luminescence (donor-acceptor luminescence); and luminescence between Cl or Al, which trapped electrons, and Zn electron holes, which trapped positive holes, is called SA luminescence (self-activated luminescence).
These are described in detail in “Phosphor Handbook,” Chapter 2, 2–7, pp. 143–157, (edited by the Phosphor Research Society (Keikotai Dogakukai), published by Ohm-sha, Ltd.; December 1987). As a simple determination method, it is useful to evaluate luminescence at a low temperature. In the case of donor-acceptor type luminescence, luminous efficiency reduces when a temperature is lowered, since luminescence occurs as a result of a recombination process involving a change in transition energy of the electrons (holes). This can be confirmed by electroluminescence, but as a simpler method, the same result is obtained with photoluminescence as well. For luminescence given in the transition in the luminescence center of impurities such as Mn and Eu, in contrast, there is a lower probability of non-radiative recombination. In general, the luminescence intensity is increased at a low temperature and a variation in thermal energy level between radiative transition states is reduced, giving a luminescence with a narrow half-value width. For the individual particles as well, the luminescence change can be examined through the observation on the luminescence condition with an optical microscope, for instance, at a room temperature and a liquid nitrogen temperature.
These electroluminescence particles are indeterminate-form particles that are usually of particle size 20 μm or more, and they are formed by a firing process (solid-phase reaction) in a high-temperature furnace at 900° C. or more. Accordingly, no generation format wherein nucleus formation and growth are separated can be taken, and further, convection of particles cannot be expected in a growing crucible, either. Therefore, the growth is easily affected by the locality of temperature or atmosphere, and the size distribution of the particles gets broader as the size becomes larger. In other words, as the growth advances more and more, large particles become larger and small particles grow more slowly. Thus, the size distribution gets broad. As a result, the distribution of luminous property between particles, gets large; and accordingly, high luminance cannot be obtained unless high voltage is applied.
Since a high-temperature process is not required to produce the particle dispersion type, this type has following advantageous characteristics: A flexible device having a plastic as a substrate can be produced, the type can be produced at low costs through relatively simple steps without using a vacuum machine, and the luminous color of the device can easily be adjusted by mixing multiple kinds of phosphor particles that give different luminous colors. Thus, this type is applied to back lights in LEDs and so on, and display devices. However, the light-emission luminance and the efficiency thereof are low, and high voltage of 100 V or more is required for high-luminance luminescence. As a result, the scope to which the particle dispersion type can be applied is restricted, and it is therefore desired to improve the light-emission luminance and the luminous efficiency further.
As a measure to increase the light-emission luminance and to decrease the voltage for luminescence, it is widely known to make the film thickness of the phosphor layer thinner, thereby making the electric field in the phosphor layer higher. Usually, however, when the phosphor particles are of size 20 μm or more, there is a problem: it is difficult to form a smooth phosphor layer with a film thickness controlled to 60 μm or less, by coating. Occurrence of unevenness (irregularities) is inevitable, and this results in deteriorated voltage-withstand performances, a shortened life, and uneven luminescence, of the devices. In contrast, it is well known that the luminance is lowered when the size of the particles is made small. In particular, when the size of particles is less than 5 μm, it is well known in the art that: the phosphor layer can be made thin, but such a thin layer is incompatible with attaining high luminance and high efficiency.
The thin-film-type device comprises, on an electrically conductive substrate, a pair of dielectric layers, and a homogeneous light-emitting layer sandwiched between the dielectric layers. These films are sub-micron thin films formed by a vacuum process. A typical example of the material for the light-emitting layer is ZnS. This is doped with ions of Mn, Cu, or the like, as luminescence centers. To these basic layers, an insulating layer, to prevent dielectric breakdown; a diffusion barrier layer, to prevent inter-layer movement of ions, and other layers, may be occasionally added.
However, this thin film type is driven by applying a high voltage to the thin laminate structure thereof; therefore, dielectric breakdown may be destructively caused in a defective site of the film. Accordingly, delicate management in the production thereof, such as protection against dust, is required. Thus, the production of a large-area device is not easy.
Accordingly, although the thin film type is suited for display devices, it is not easily made into a large area and it is difficult to simplify its production process. Thus, a uniform large-area light source using this type has not been realized.